Spectrum analyzer and method for correcting frequency errors

Abstract

A spectrum analyzer corrects for internal frequency errors in a reference oscillator using a timing control signal. The reference oscillator provides a reference signal at a reference frequency. An error detection circuit determines an error in the reference frequency using the timing control signal and produces an error correction signal for use by a frequency conversion device in adjusting an output frequency thereof to compensate for the frequency error in the reference frequency.

1. A spectrum analyzer, comprising:
 a reference oscillator for providing a reference signal at a reference frequency; 
 an error detection circuit connected to receive said reference signal and a timing control signal and operable to determine an error in said reference frequency using said timing control signal and produce an error correction signal for use in correcting said error; 
 a frequency conversion device connected to receive said error correction signal and said reference signal and is operable to produce an output frequency using said error correction signal to compensate for said error in said reference signal; 
 an additional frequency conversion device connected to receive a radio frequency (RF) signal and a local oscillation signal produced from said reference signal and operable to convert said RF signal to an intermediate frequency (IF) signal using said local oscillation signal; 
 an analog-to-digital converter connected to receive said IF signal and operable to convert said IF signal to a digital signal; and 
 a processor including said frequency conversion device and connected to receive said digital signal and said error correction signal; 
 wherein said processor is operable to process said digital signal using said error correction signal. 
 
   
   
     2. The spectrum analyzer of  claim 1 , wherein said frequency conversion device includes a local oscillator connected to receive said reference signal and said error correction signal, wherein said local oscillator is operable to convert said reference signal at said reference frequency to a local oscillation signal at a measurement frequency. 
   
   
     3. The spectrum analyzer of  claim 2 , wherein said local oscillator includes a voltage controlled oscillator connected to receive a control voltage produced from said error correction signal and operable to produce said local oscillation signal based on said control voltage. 
   
   
     4. The spectrum analyzer of  claim 3 , wherein said local oscillator further includes a phase and frequency detector connected to receive said reference signal and a feedback signal and operable to produce an error signal indicative of a difference in phase or frequency between said reference signal and said feedback signal, and a voltage generator connected to receive said error signal and said error correction signal and operable to generate said control voltage proportional to a combination of said error signal and said error correction signal. 
   
   
     5. The spectrum analyzer of  claim 4 , wherein said voltage controlled oscillator is further operable to produce said feedback signal as said local oscillation signal, and wherein said local oscillator further includes a frequency divider connected to receive said feedback signal and divide said feedback signal by a divide ration, and wherein said feedback signal generated by said voltage controlled oscillator is said local oscillation signal having said measurement frequency equal to the product of said reference frequency of said reference signal and said divide ratio. 
   
   
     6. The spectrum analyzer of  claim 1 , wherein said frequency conversion device includes a frequency adjustment component operable to adjust a frequency of said digital signal as a function of said error correction signal to produce a corrected digital signal. 
   
   
     7. The spectrum analyzer of  claim 6 , wherein said frequency adjustment component includes a summation node connected to combine said digital signal with said error correction signal to produce said corrected digital signal. 
   
   
     8. The spectrum analyzer of  claim 1 , wherein said timing control signal includes timing signals separated by known time intervals and wherein said error detection circuit includes:
 a counter circuit connected to receive said reference signal and operable to count cycles of said reference signal occurring between successive ones of said timing signals to produce a count value indicative of said reference frequency; and 
 a processor connected to receive said count value and operable to determine said reference frequency from said count value, determine said error as a difference between said reference frequency and a desired frequency and produce said error correction signal to compensate for said error in said reference frequency. 
 
   
   
     9. The spectrum analyzer of  claim 8 , wherein said timing signals are pulses of a Global Positioning System (GPS) clock signal generated at a rate of one pulse per second. 
   
   
     10. The spectrum analyzer of  claim 1 , wherein said error detection circuit includes:
 a phase comparator connected to receive said reference signal and said timing control signal and operable to detect a phase difference between said reference signal and said timing control signal; and 
 a processor connected to receive said phase difference and operable to determine said error from said phase difference and produce said error correction signal to compensate for said error in said reference frequency. 
 
   
   
     11. The spectrum analyzer of  claim 1 , further comprising:
 an inbound signal processing portion connected to receive said output frequency signal, said inbound signal processing portion operable for using said output frequency to support frequency spectrum analysis of an inbound signal. 
 
   
   
     12. A method for correcting frequency errors in a spectrum analyzer, said method comprising:
 receiving a reference signal at a reference frequency; 
 determining an error in said reference frequency using a timing control signal; 
 producing an error correction signal for use in correction said error; 
 producing an output frequency by a frequency conversion device that uses said error correction signal and said reference signal to compensate for said error in said reference signal; 
 receiving a radio frequency (RF) signal; 
 converting said RF signal to an intermediate frequency (IF) signal using a local oscillation signal produced from said reference signal; 
 converting said IF signal to a digital signal; and 
 processing said digital signal using said error correction signal. 
 
   
   
     13. The method of  claim 12 , further comprising:
 converting said reference signal at said reference frequency to a local oscillation signal at a measurement frequency. 
 
   
   
     14. The method of  claim 13 , wherein said converting further comprises:
 producing a control voltage from said error correction signal; and 
 producing said local oscillation signal based on said control voltage. 
 
   
   
     15. The method of  claim 14 , wherein said producing said control voltage further includes:
 receiving said reference signal and a feedback signal produced from said local oscillation signal; 
 producing an error signal indicative of a difference in phase or frequency between said reference signal and said feedback signal; and 
 generating said control voltage proportional to a combination of said error signal and said error correction signal. 
 
   
   
     16. The method of  claim 13 , further comprising:
 receiving a radio frequency (RF) signal; and 
 convening said RF signal to an intermediate frequency (IF) signal using said local oscillation signal. 
 
   
   
     17. The method of  claim 12 , wherein said processing further includes:
 adjusting a frequency of said digital signal as a function of said error correction signal to produce a corrected digital signal. 
 
   
   
     18. The method of  claim 12 , wherein said timing control signal includes timing signals separated by known time intervals and wherein said producing said error correction signal further includes:
 counting cycles of said reference signal occurring between successive ones of said timing signals to produce a count value indicative of said reference frequency; 
 determining said reference frequency from said count value; 
 determining said error as a difference between said reference frequency and a desired frequency; and 
 producing said error correction signal to compensate for said error in said reference frequency. 
 
   
   
     19. The method of  claim 12 , wherein said producing said error correction signal further includes:
 detecting a phase difference between said reference signal and said timing control signal; 
 determining said error from said phase difference; and 
 producing said error correction to signal to compensate for said error in said reference frequency. 
 
   
   
     20. The method of  claim 9 , further comprising:
 using said output frequency to support frequency spectrum analysis of an inbound signal.